This invention relates to power transmission systems utilizing toothed power transmission belts cooperating with toothed pulleys, and also relates to belts and pulleys for use in such a system.
Power transmission systems are known utilizing belts having a plurality of alternating teeth and grooves extending generally transversally of the belt and meshing with alternating teeth and cavities in a grooved pulley in order to perform a driving function. Many such belt and pulley systems are currently in use and considerable work has been done in designing the optimum shapes for the cross-sections for the teeth and grooves on both the belt and the pulley. For example, U.S. Pat. No. 3,756,091 to Miller discloses a positive drive system in which the belt includes relatively closely spaced together driving teeth of a special curvilinear cross-sectional configuration which is operated in conjunction with mating pulley cavities defined between pulley teeth having curvilinear cross-sections which are substantially conjugate to the teeth of the belt. Given a toothed belt in combination with a pulley, the conjugate form of the belt tooth is that tooth form described on the pulley which corresponds to the volume between the belt and the pulley which is not swept out by the belt tooth as the belt moves into contact with the pulley. Use of conjugate teeth on the pulley is desirable as it insures that the belt and pulley teeth are able to engage and disengage without interference.
Belts having substantially curvilinear teeth in accordance with the Miller patent resulted in reduced belt tooth shear and increased horsepower capacity. While the toothed power transmission belt and pulley system of the Miller patent has proven valuable for many commercial applications, particularly for high torque drive applications, the almost vertical tooth flank and the substantial depth of the teeth of the Miller belt require that it be used with a pulley cavity that is substantially wider than the belt tooth. It is this clearance, inherent in the Miller construction, that results in backlash. The precision of indexing, or positioning, drives is controlled by the backlash or clearance between the belt tooth flanks and the cooperating pulley cavity flanks. None of the toothed belt and pulley power transmission systems known today are capable of providing, particularly at high loads, the precision indexing required in such equipment as high-speed printers, machine tool positioners, or computer controlled assembly robots.
It will be seen that while it may be possible to take belts of known construction and increase the belt tooth width dimension and/or have such belts cooperate with pulleys in which the cavity width dimension is decreased, such constructions, while possibly reducing backlash, would result in substantially reduced belt life, increased noise, and/or require more driving power because they would necessarily result in meshing interference between belt tooth and pulley cavity. The minimum clearance required to prevent entry or exit meshing interference for any belt tooth design can be found by rolling the belt tooth on the pitch diameter of the smallest pulley with which it is intended to be used. The profile that the belt tooth sweeps out as it enters and exits the pulley defines the minimum pulley cavity profile (i.e. conjugate pulley cavity) with which the belt tooth will mesh without interference.
The Hoback patent, U.S. Pat. No. 4,037,485, discloses a power transmission system which, when the belt thereof is put under sufficient tension the material of the individual belt teeth spreads within the pulley cavities so as to substantially fill those cavities and thereby substantially reduce backlash. However, such a construction has been found not to be practical since the high tension required and the resulting interference between the belt teeth and pulley cavities, when meshing, result in accelerated wear of the belt jacket material so as to substantially reduce the operational life thereof. Furthermore, the high tension which is required by Hoback results in a loss of useful power and necessitates a strengthening and overdesigning of the structural components of the system. All of these disadvantages are overcome by the system according to the present invention.